report undefined operator instead of method for bad +=.

Change-Id: Ia28fdb47c80768b011b05edc1e3e7b832f9335e8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/105971
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Mike Fairhurst <mfairhurst@google.com>
diff --git a/pkg/analyzer/lib/src/generated/element_resolver.dart b/pkg/analyzer/lib/src/generated/element_resolver.dart
index 27097db..0ab26fe 100644
--- a/pkg/analyzer/lib/src/generated/element_resolver.dart
+++ b/pkg/analyzer/lib/src/generated/element_resolver.dart
@@ -169,7 +169,7 @@
         if (_shouldReportInvalidMember(staticType, staticMethod)) {
           _recordUndefinedToken(
               staticType.element,
-              StaticTypeWarningCode.UNDEFINED_METHOD,
+              StaticTypeWarningCode.UNDEFINED_OPERATOR,
               operator,
               [methodName, staticType.displayName]);
         }
diff --git a/pkg/analyzer/test/generated/static_type_warning_code_test.dart b/pkg/analyzer/test/generated/static_type_warning_code_test.dart
index 655d38e..ad2f86f 100644
--- a/pkg/analyzer/test/generated/static_type_warning_code_test.dart
+++ b/pkg/analyzer/test/generated/static_type_warning_code_test.dart
@@ -1732,7 +1732,7 @@
   }
 }
 ''', [
-      error(StaticTypeWarningCode.UNDEFINED_METHOD, 58, 2),
+      error(StaticTypeWarningCode.UNDEFINED_OPERATOR, 58, 2),
     ]);
   }